Abstrict The present specification discloses a hot wire air flow meter with
high circuit stability. The hot wire type air flow meter comprises
a hot wire located in an air flow path to be measured and to which
current is supplied from the external, a circuit for detecting a
value of current flowing through the hot wire, an amplifying circuit
including an inverting input terminal, a non-inverting input terminal
to which a value detected in the current detecting circuit is applied,
and an output terminal to which an output of an air-flow function
is applied, a feedback circuit for supplying a feedback output to
any terminal except the non-inverting input terminal of the amplifying
circuit and controlling the output of the amplifying circuit to
keep the temperature of the hot wire stable according to the output
of the amplifying circuit, and a circuit for compensating for a
phase of an output of the amplifying circuit contained in the feedback
circuit.
Claims We claim:
1. A hot wire type air flow meter, comprising:
a hot wire to be located in an air flow path and to which a heating
current is to be externally supplied;
means connected to said hot wire for detecting a value of heating
current flowing through said hot wire;
amplifying means having an inverting input terminal (-), and a
non-inverting input terminal (+) to which an output of said current
detecting means is applied, for applying to an output terminal thereof
a signal which is a function of air flow in the air flow path;
feedback means connected to supply a feedback output related to
said heating current to said inverting input terminal of said amplifying
means for controlling the output of said amplifying means to keep
the temperature of said hot wire constant according to the output
of said amplifying means; and
means connected to said amplifying means and isolated from said
detecting means for compensating for a phase of an output of said
amplifying means;
wherein a buffer is provided between said feedback means and said
compensating means.
2. A hot wire type air flow meter according to claim 1 further
including a temperature compensating circuit having a thermal resistor
to be located in said air flow path and responsive to an output
of said amplifying means for compensating for a temperature of air
to be measured.
3. A hot wire type air flow meter according to claim 2 wherein
said phase compensating means is connected to the output terminal
of said amplifying means.
4. A hot wire type air flow meter according to claim 2 wherein
said phase compensating means is connected to said temperature compensating
circuit.
5. A hot wire type air flow meter according to claim 2 wherein
said feedback means comprises a voltage divider circuit which is
connected to the inverting input terminal of said amplifying means.
6. A hot wire type air flow meter, comprising:
a hot wire to be located in an air flow path;
means for supplying a heating current to one end of said hot wire;
means connected to the other end of said hot wire for detecting
a value of heating current flowing through said hot wire;
control means, including an amplifier having a non-inverting input
terminal to which an output of said heating current detecting means
is connected, an inverting input terminal, and an output terminal,
for controlling said heating current supplying means to keep the
temperature of said hot wire constant;
feedback means connected to supply a current, which represents
a level of said heating current, from said one end of said hot wire
to said inverting input terminal of said amplifier;
compensating means connected to the output terminal of said amplifier
for compensating for a phase of an output of said amplifier;
a temperature compensating circuit, having a thermal resistor to
be located in said air flow path, connected between said output
terminal of said amplifier and ground for compensating for the temperature
of air to be measured.
7. A hot wire type air flow meter according to claim 6 wherein
said heating current supplying means includes a control transistor
for coupling a source of heating current to one end of said hot
wire, and wherein said feedback means comprises a circuit including
at least one resistor connected between said one end of said hot
wire and said inverting input terminal of said amplifier.
8. A hot wire type air flow meter according to claim 7 wherein
said feedback means comprise a series circuit of first and second
non-thermal resistors.
9. A hot wire type air flow meter comprising:
a hot wire to be located in an air flow path;
means for supplying a heating current to said hot wire, including
a control transistor for coupling a source of heating current to
one end of said hot wire;
means connected to said hot wire for detecting a value of heating
current flowing through said hot wire;
control means, including an amplifier having a non-inverting input
terminal to which an output of said heating current detecting means
is connected, an inverting input terminal, and an output terminal,
for controlling said heating current supplying means to keep the
temperature of said hot wire constant;
feedback means, including a series circuit of first and second
resistors, connected to supply a current, which represents a level
of said heating current, to said inverting input terminal of said
amplifier;
compensating means connected to said amplifier for compensating
for a phase of an output of said amplifier; and
wherein said control means comprises a further amplifier having
an inverting input terminal connected to a point of connection of
said first and second resistors, a non-inverting input terminal
connected to an output terminal of the first-mentioned amplifier
and an output terminal connected to a control electrode of said
control transistor.
10. A hot wire type air flow meter according to claim 9 wherein
compensating means, which comprises a circuit including at least
one resistor and a capacitor, is connected between said output terminal
of said amplifier and ground.
11. A hot wire type air flow meter according to claim 10 further
including a temperature compensating circuit, having a thermal resistor
to be located in said air flow path, connected between said output
terminal of said amplifier and ground for compensating for the temperature
of air to be measured.
12. A hot wire type air flow meter according to claim 11 wherein
said compensating means is connected to said temperature compensating
circuit.
13. A hot wire type air flow meter according to claim 11 wherein
said compensating means is connected in parallel with said temperature
compensating circuit.
14. A hot wire type air flow meter according to claim 9 further
including a temperature compensating circuit, having a thermal resistor
to be located in said air flow path, connected between said output
terminal of said amplifier and ground for compensating for the temperature
of air to be measured.
15. A hot wire type air flow meter comprising:
a hot wire to be located in an air flow path;
means for supplying a heating current to said hot wire;
means connected to said hot wire for detecting a value of heating
current flowing through said hot wire;
control means, including an amplifier having a non-inverting input
terminal to which an output of said heating current detecting means
is connected, an inverting input terminal, and an output terminal,
for controlling said heating current supplying means to keep the
temperature of said hot wire constant;
feedback means connected to supply a current, which represents
a level of said heating current, to said output terminal of said
amplifier; and
compensating means connected to said amplifier for compensating
for a phase of an output of said amplifier; and
wherein said heating current supplying means includes a control
transistor for coupling a source of heating current to one end of
said hot wire, and wherein said feedback means comprises a series
circuit of first and second resistors connected between said one
end of said hot wire and said output terminal of said amplifier.
16. A hot wire type air flow meter according to claim 15 wherein
said control means comprises a further amplifier having an inverting
input terminal connected to a point of connection of said first
and second resistors, a non-inverting input terminal connected to
an output terminal of the first-mentioned amplifier and an output
terminal connected to a control electrode of said control transistor.
17. A hot wire type air flow meter according to claim 15 wherein
said compensating means comprises a circuit including at least one
resistor and a capacitor connected between said output terminal
of said amplifier and ground.
18. A hot wire type air flow meter according to claim 17 further
including a temperature compensating circuit, having a thermal resistor
to be located in said air flow path, connected between said output
terminal of said amplifier and ground for compensating for the temperature
of air to be measured.
Description BACKGROUND OF THE INVENTION
The present invention relates to a control circuit for a constant
temperature type hot wire air flow meter.
FIG. 1 is a circuit diagram showing one example of the conventional
constant temperature type hot wire air flow meter. In the circuit,
1 denotes a hot wire made of platinum or the like. The hot wire
1 is controlled to be a constant temperature by a circuit consisting
of operational amplifiers 5 6 a transistor 7 and resistors 3
9 21 22. Air flow directed at the hot wire 1 results in a change
in its resistance value. To cope with the change, the circuit serves
to control the resistance value to be constant (the constant resistance
value means a constant temperature). At a given time, a voltage
is generated across resistor 3. This voltage represents a function
of the air flow rate so that it can be used to produce a signal
which is a measure of the air flow rate. A resistor 2 is a cold
wire used for compensating for the temperature of the air to be
measured.
FIG. 2 is an equivalent circuit of an air flow meter shown in FIG.
1. Resistors 1 and 3 in FIG. 2 are arranged in the same manner as
those shown in FIG. 1. An amplifier 56 is an equivalent amplifying
circuit composed of operational amplifiers 5 6 resistors 2 9
21 22 and the like. A voltage 256 is a d.c. voltage value defined
by the circuit arrangement. This value is an index representing
how much the circuit operation shifts out of a linear operation.
It can be represented as follows (this value is referred to as a
general offset voltage).
wherein V.sub.2 is a voltage which appears at a branch point between
the resistors 1 and 3 V.sub.1 is an output of the amplifier 56
and G is a gain of the amplifier 56.
Further, the voltages V.sub.1 and V.sub.2 have the following relation;
From the equations (1) and (2), the voltages V.sub.1 and V.sub.2
can be derived as follows. ##EQU1## As will be understood from the
equations (3) and (4), no value of V.sub.1 and V.sub.2 can be provided
without a value of V.sub.OF.
That is, the subject circuit provides a general offset voltage
V.sub.OF, that is, an element defining the circuit operation in
itself. By designing this value properly, the air flow meter reaches
an optimum operation.
The value of V.sub.OF is defined by an input offset voltage V.sub.OPF
of the operational amplifiers 5 and 6 resistors 24 and 28 a voltage
V.sub.CC and the like. To keep the value of V.sub.OF constant, an
external circuit composed of resistors 24 28 and the like may be
employed.
Even if the external circuit is used for keeping the value of V.sub.OF
constant, this value will inevitably vary because it is subject
to various factors such as temperature and power supply voltage
level. In particular, since, V.sub.3.sup.OF.sub.2 is likely to be
affected by the temperature and since the temperature is likely
to vary widely, the value of V.sub.OF under certain conditions may
become zero or lower than zero. This results often in a disabling
of the circuit shown in FIG. 1 as an air flow meter; bringing it
into an oscillating state.
One example of the conventional hot wire air flow meter has been
disclosed in the Un-examined Patent Publication JP-A-64-8828 of
the Japanese Application Serial No. 62-244096 Hitachi, Ltd., filed
on Nov. 30 1987.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a control
circuit for controlling an air flow meter, which control circuit
can be operated stably even if the general offset voltage V.sub.OF
becomes zero or less than zero.
To achieve the foregoing object, the control circuit includes a
phase compensating circuit and a feedback circuit which is effective
for stabilizing the input offset voltage V.sub.OF of the operational
amplifier which would be otherwise amplified by the control circuit
itself.
The combination of the phase compensating circuit and the feedback
circuit functions to stabilize the control circuit in processing
a signal representing an air flow rate. It is, therefore, expected
that the control circuit operates stably without entering a state
of oscillation when the offset voltage V.sub.OF is 0 V or less than
0 V.
This stable operation of the control circuit is the result of two
causes, that is, allowing the phase compensating circuit to compensate
for a signal phase and stabilizing the offset voltage V.sub.OF defined
by the control circuit, which voltage would be otherwise amplified
by the circuit itself.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a circuit diagram showing a control circuit of a conventional
hot wire air flow meter;
FIG. 2 is a circuit diagram showing an equivalent circuit of the
circuit shown in FIG. 1;
FIG. 3 is a circuit diagram showing a control circuit of an air
flow meter;
FIG. 4 is a graph showing the characteristics of the circuits shown
in FIGS. 1 and 3; and
FIGS. 5 6 and 7 are circuit diagrams showing an air flow meter
according to other embodiments of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
One embodiment of the present invention will be described with
reference to FIG. 3. As shown, similar components to those of the
prior art are referenced by similar numbers. A phase compensating
circuit is composed of resistors 17 18 and a capacitor 19 and serves
to compensate for a phase of a flow rate signal in a specific time
area (that is, a frequency area).
A feedback circuit for the operational amplifier 6 is composed
of resistors 21 and 22. Unlike the feedback contact point at the
end of resistor 22 as shown in FIG. 1 the feedback contact is not
connected to a non-inverting input terminal (+) of the operational
amplifier 5 but is connected to an inverting input terminal (-)
thereof. In this connection, the control circuit itself does not
function to amplify an input offset voltage V.sub.OPF1 (d.c. voltage
between (+) and (-) input terminals) of the operational amplifier
5. It results in a stabilizing of the control circuit.
FIG. 4 shows a characteristic of the embodiment shown in FIG. 3.
In the characteristic graph, the x-axis represents the general offset
voltage V.sub.OF and the y-axis represents stability of the sensor.
As will be understood from this graph, the conventional circuit
shown in FIG. 1 has to keep a value of V.sub.OF as a proper value,
because the circuit enters into an unstable area as shown by a curve
l.sub.0. However, the air flow meter shown in FIG. 3 employing the
invention is designed to improve a stability curve as l.sub.1 as
shown in FIG. 4 because the air flow meter of this invention functions
to apply a feedback signal to a non-inverting input terminal and
to compensate for a phase of an output of the operational amplifier.
As a result, there appears an area where the control circuit is
stably operated even when the offset voltage V.sub.OF becomes negative.
This function was found out by the present inventors.
FIG. 5 shows another embodiment of the invention. According to
this embodiment, the phase compensating circuit is composed of a
resistor 18 a capacitor 19 and resistors 22 23. This circuit arrangement
is effective for replacing the function of the resistor 17 of the
phase compensating circuit shown in FIG. 3 by the resistors 22 and
23. It results in reducing the number of resistors required in the
circuit.
FIG. 6 shows another embodiment of the invention. In this embodiment,
a resistor is connected to an output terminal of the operational
amplifier 5. The operational amplifier provides so low a resistance
value that the resistance values of the resistors 21 22 and 23
can be easily designed.
FIG. 7 shows the other embodiment of the invention. In this embodiment,
an operational amplifier 40 is newly provided and has an output
end to which one end of the resistor 22 is fed back. Further, this
embodiment is designed to separate phase compensating circuits 17
18 and 19 from feedback circuits 21 22 through an operational amplifier
40 provided between the circuits 17 18 and the resistors 21 and
22. Hence, it can offer more stability because no interference is
caused between the compensating level given by the compensating
circuit and the feedback level given by the feedback circuit.
According to the invention, the phase compensating circuit and
the feedback circuit are compositely operated so as to treat a signal.
It results in being able to operate the air flow meter stably even
when the general offset voltage is negative.
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